FIG. 1 illustrates a prior art energy conversion system in which an array of photovoltaic (PV) panels 10 are connected in series string to generate a DC bus voltage VA+,VA− from the array. The DC bus voltage is converted to AC power in the form of an AC voltage VAC by an inverter 12.
Referring to FIG. 2, curve 16 illustrates the voltage-current characteristic (V-I curve) of a typical PV panel under certain operating conditions, while curve 18 illustrates the corresponding power characteristic (power curve) for the same panel under the same conditions. The V-I curve is zero volts with a value of ISC which is the short-circuit current generated by the panel when the output terminals are shorted together. As the output voltage increases, the V-I curve remains at a fairly constant level of current until it reaches a knee at which it descends rapidly toward zero current at VOC, which is the open-circuit output voltage of the panel.
The power curve is simply the current times the voltage at each point along the V-I curve. The power curve has a maximum value corresponding to a certain voltage level and a certain current level. This is known as the maximum power point or MPP. The maximum power point tends to change based on changes in operating conditions such as illumination level, temperature, age of the panel, etc.
Inverters for PV systems typically include maximum power point tracking (MPPT) functionality that attempts to operate the system at the maximum power point. Since the maximum power point changes based on changes in operating conditions, algorithms have been devised for tracking the MPP as it changes over time. Conventional MPPT algorithms often employ a perturb and observe technique in which the operating voltage or current is varied, thereby causing the operating point to vary around the MPP as shown in FIG. 2. By observing the change in power as the voltage or current changes, the MPP can be determined.
In systems that only include a single PV panel, existing MPPT algorithms enable the system to operate at the true maximum power point for the single panel. In systems having multiple PV panels as shown in FIG. 1, existing MPPT algorithms 14 attempt to find the average maximum power point for the overall system. However, finding the average maximum power point for the system does not necessarily enable each individual panel to operate at its maximum power point. By not operating each panel at its maximum power point, the energy produced by the system may be reduced resulting in lost income and a longer return on investment. This loss of efficiency may be especially problematic with PV arrays that are subjected to shading in part of the array because the reduction in efficiency is often disproportionately larger than the area that is shaded.